// Application to vertex shader
  varying vec3 N;
  varying vec3 I;
  varying vec4 Cs;

  void main()
  {
          vec4 P = gl_ModelViewMatrix * gl_Vertex;
          
          I  = P.xyz - vec3 (0);
          
          N  = gl_NormalMatrix * gl_Normal;
          
          Cs = gl_Color;
          
          gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;    
  }